A plurality of sensor elements and methods for sensing at least one property of a measured gas in a measured gas space are conventional. Any physical and/or chemical properties of the measured gas can, in principle, be involved, and one or more properties can be sensed. The present invention is described below in particular with reference to a qualitative and/or quantitative sensing of one portion of a gas component of the measured gas, in particular with reference to sensing of an oxygen component in the measured gas part. The oxygen component can be sensed, for example, in the form of a partial pressure and/or in the form of a percentage. Alternatively or additionally, however, other properties of the measured gas can also be sensed, for example the temperature.
Such sensor elements can be configured, for example, as so-called lambda probes, as are described in Konrad Reif (ed.): Sensoren im Kraftfahrzeug (Sensors in motor vehicles), 1st edition 2010, pp. 160-165. Using broadband lambda probes, in particular planar broadband lambda probes, for example, the oxygen concentration in the exhaust gas can be determined over a wide range, and the air-fuel ratio in the combustion chamber can thereby be inferred. The air ratio λ describes this air-fuel ratio.
Ceramic sensor elements that are based on the utilization of electrolytic properties of specific solids, i.e., on ion-conduction properties of those solids, are available. These solids can in particular be ceramic solid electrolytes such as, for example, zirconium dioxide (ZrO2), in particular yttrium-stabilized zirconium dioxide (YSZ) and scandium-doped zirconium dioxide (ScSZ), which can contain small additions of aluminum oxide (Al2O3) and/or silicon oxide (SiO2).
Despite the advantages of the conventional sensor elements and methods for diagnosis, they still have the potential for improvement. For example, an interruption in the lead to virtual ground is usually detected based on the conditions of an implausibly high resistance of the solid electrolyte layer and a pump current of 0 A. This is because in the context of an interruption in the lead to virtual ground, the measurement path for determining the probe's internal resistance is interrupted. The indicated internal resistance is consequently very high even though the probe may be sufficiently hot. The pump current criterion of 0 A results from the fact that the pump current cannot flow off via the lead to virtual ground. Because the system cannot distinguish a cold probe from an interruption in the lead to virtual ground, it is necessary to wait a while after starting before enabling diagnosis. The so-called “pinpointing” effect correspondingly exists. A failed lead to virtual ground and a heating power level fault have the same fault symptoms. In both cases, for example, the measured internal resistance is implausibly high and the pump current is 0 A, so that the two faults can be difficult to distinguish. The system cannot distinguish a cold probe from an interruption in the lead to virtual ground; hence the need for the waiting time (i.e., heating phase) after engine starting before diagnosis can be enabled. Because the measured internal resistance does not correlate with the actual resistance in the event of a fault, the heating regulator requests maximum heating power with no rise in the indicated internal resistance value. Especially in the case of a probe having a high-power heating element, this can result in overheating of the probe. Probe operational readiness is set when the probe temperature exceeds a threshold or the internal probe resistance falls below a threshold. In the event of a fault, the indicated internal probe resistance drifts downward due to thermal effects after a certain time, and a value of approximately 1 kilohm is established. For many types of probe, however, this value is already below the threshold for operational readiness. The result of this is that the lambda controller switches on even though a valid lambda signal is not available due to the failure in the virtual ground lead. This can cause the engine to run roughly.